1. Field of the Invention
The present invention relates, in general, to a piezoelectric ceramic composition reducing a leakage current and a piezoelectric device using the same and, in particular, to a piezoelectric ceramic composition in which a B-site of the PZT piezoelectric ceramic composition is substituted with a certain material, and chromium oxide is added, and a piezoelectric device using the piezoelectric ceramic composition. The piezoelectric ceramic composition has good heat resistance and frequency stability, and reduces a leakage current.
2. Description of the Prior Art
A recent piezoelectric sensor is widely used in a ceramic resonator, a ceramic filter, a piezoelectric transducer, a piezoelectric buzzer, an ultrasonic oscillator, or the like. For example, a kHz-range SMD (surface mounted device) type filter, which is an essential part in recent information technology, is commonly used to produce two layered IF filters in devices such as a pager, AMPS (Advanced Mobile Phone Service System), wireless transmitting/receiving apparatus, wire/wireless hands-free kit, and wireless network. Use of the piezoelectric sensor is continuously expanding, and more advanced electrical/piezoelectric properties of the piezoelectric sensor are required according to developments in sensitivity of piezoelectric sensors.
According to the great advances in the information technology, demand for a light weight, small-sized, and highly efficient electronic material and frequency stability of the electronic material is increased.
Particularly, when SMD type chip parts are used, they should be passed through a reflow process. Accordingly, a material is required which has piezoelectric properties unaffected by the reflow process, and resistance to high temperatures.
However, conventional piezoelectric compositions have low heat resistance owing to low Tc (phase transition temperature), and changes in their piezoelectric properties and frequency stability after the reflow process are large, so that a high value-added SMD type filter can hardly be produced from conventional piezoelectric compositions. Also, various electronic products cannot be produced and a competitive power of products is poor because the piezoelectric properties of the conventional piezoelectric composition are difficult to control.
To secure the heat resistance of the material and stabilize frequency and the piezoelectric properties of the material after the reflow process, studies have been conducted for stabilizing a heat resistance, as well as a frequency change, a capacitance change, and a coupling factor (k) change after the reflow process according to a oscillation mode by using a piezoelectric composition comprising Pb(Zr, Ti) O3 as a main component and additives such as Mn, Y, Dy, Er, Ho, Lu, and Yb.
For example, U.S. Pat. No. 6,123,867 discloses a piezoelectric ceramic composition comprising Pb[(M, Nb) Zr, Ti]O3 as a main component and additives, and a piezoelectric device using the piezoelectric ceramic composition, which can stabilize a heat resistance, as well as a frequency change ratio, a capacitance change ratio, and a coupling factor (k) change ratio after the reflow process.
Furthermore, a Pb(Zn Nb) (Sn, Nb) TiZrO3 based composition disclosed in Japanese Patent Publication Nos. Sho. 52-17239 and Sho. 51-7318 and a Pb(Sn, Sb)TiZrO3 based composition disclosed in Japanese Patent Publication Nos. Sho. 54-32516 and Sho. 54-36757 are suitable for use in a high frequency ceramic oscillator or filter, because of their excellent piezoelectricity and small size of crystal grains, as well as ease in removing impurities during sintering thereof. Therefore, these piezoelectric ceramic compositions are used in ceramic filters, ceramic oscillators, piezoelectric transducers, ceramic sensors or the like.
Additionally, Japanese Laid-Open Patent Publication Nos. Hei. 8-239269 and Hei. 9-142930 disclose materials that are not affected by heat, in which a composite oxide of Y, Nb or the like is the main component of PZT, or Cr is added thereto.
However, the characteristics of the conventional piezoelectric ceramic composition change because of poor heat resistance, and drift occurs upon heating. Therefore, the piezoelectric device has poor reliability and disadvantages in mass production. In other words, when a piezoelectric device formed of the conventional piezoelectric ceramic composition is heated at 150xc2x0 C. for 1 hour, the resonance frequency fr changes by several % immediately after the heat treatment, when compared to that before the heat treatment.
Additionally, a reflow temperature is higher and a temperature of an element heated is also higher because SMT type electronic parts such as ceramic filters and ceramic oscillators are commonly used. Accordingly, the piezoelectric properties of the piezoelectric ceramics before the heat treatment are different from those after the heat treatment. That is to say, there is a problem that the piezoelectric propertiesxe2x80x94such as resonance frequencyxe2x80x94of the piezoelectric ceramics before the heat treatment are different from those after the heat treatment, when such piezoelectric ceramics are heated to a temperature of about 250xc2x0 C. and then cooled to room temperature in order to reflow the piezoelectric device.
Furthermore, there is another problem that a leakage current becomes large and thus a voltage is nonuniform when the conventional piezoelectric composition is polarized, therefore dispersion is large and productivity is poor while the piezoelectric device is produced.
Therefore, it is an object of the present invention to provide a piezoelectric ceramic composition with an excellent heat resistance and frequency stability, which is low in leakage current.
It is another object of the present invention to provide the piezoelectric ceramic composition which has a minimum dispersion owing to a reduction of the leakage current and which can be manufactured to piezoelectric devices at high production yield.
It is yet another object of the present invention to provide a piezoelectric device using the piezoelectric ceramic composition, in which the piezoelectric ceramic composition has an excellent heat resistance and frequency stability and reduces a leakage current.
According to an embodiment of the present invention, a piezoelectric ceramic composition is provided, which comprises a main component expressed by Pb[(Co1/2W1/2)xTi1xe2x88x92xxe2x88x92yZry]O3 (wherein, 0.001xe2x89xa6xxe2x89xa60.04, and 0.35xe2x89xa6yxe2x89xa60.55), Cr2O3 in an amount of 0.01 to 2 wt %, MnO2 in an amount of 0.1 to 0.5 wt %, and an additive in an amount of 0.0l to 2.0 wt %, based on the total weight of the piezoelectric ceramic composition. The additive is selected from the group consisting of CoO, MgO, ZnO, Al2O3, Fe2O3, Sb2O3, SnO2, CeO2, Nb2O5, V2O5 and WO3, and mixtures thereof.
According to another embodiment of the present invention, a piezoelectric device based on the piezoelectric ceramic composition of the present invention is provided.
Hereafter, a detailed description of the present invention will be provided.
A composite oxide, a main component of the piezoelectric ceramic composition according to the present invention is a PZT based composite oxide, a B-site of which is partially substituted with cobalt and tungsten, and which can be expressed by formula 1, below.
Pb [(Co1/2W1/2)xTi1xe2x88x92xxe2x88x92yZry]O3xe2x80x83xe2x80x83Formula 1
wherein, 0.001xe2x89xa6xxe2x89xa60.04, and 0.35xe2x89xa6yxe2x89xa60.55
In addition, the piezoelectric ceramic composition comprises Cr2O3 in an amount of 0.01 to 2 wt %, MnO2 in an amount of 0.1 to 0.5 wt %, and an additive in an amount of 0.01 to 2.0 wt %, based on the total weight of the piezoelectric ceramic composition, in which the additive is selected from the group consisting of CoO, MgO, ZnO, Al2O3, Fe2O3, Sb2O3, SnO2, CeO2, Nb2O5, V2O5 and WO3, and mixtures thereof.
Mostly comprising Pb (Ti, Zr)O3, PZT based (ABO3 composite perovskite) ceramic composition is generally applied to a high frequency ceramic oscillator and a filter owing to an excellent piezoelectricity and a small particle size.
To broaden the ranges of temperature stability and a piezoelectricity according to a variation of frequency, the PZT based ceramic composition, the B-site of which is partially substituted with (Co1/2W1/2), is used as the main component of the piezoelectric ceramic composition according to the present invention. (Co1/2W1/2) is an antiferroelectric material, which has a relatively high phase transition temperature.
The B-site of the PZT based ceramic composition is partially substituted with (Co1/2W1/2), so that a drop in the phase transition temperature is reduced, and so a loss of heat resistance according to a reduction of Tc is prevented, because Tc remains high.
In formula 1, (Co1/2W1/2) is added to the piezoelectric ceramic composition in such a way that x ranges from 0.001 to 0.04, preferably 0.025 to 0.035. For example, if x deviates from the above range, Kp value is too low or high, and thus a desired bandwidth value is not satisfied. Also, when too much Co or W are added, the piezoelectric ceramic composition is reduced in heat resistance because the Tc of the composition becomes low.
Additionally, in formula 1, Zr is added to the piezoelectric ceramic composition in such a way that y ranges from 0.35 to 0.55, preferably 0.46 to 0.50. For example, when Zr content deviates from the above range, Zr deviates from a MPB region, and thus TCF value is not satisfied and a piezoelectric effect is not sufficiently exerted because Kp value is too low.
As described above, 0.01 to 2 wt %, preferably 1 to 2 wt % of Cr2O3 based on the total weight of the piezoelectric ceramic composition is added to the main component, i.e. the PZT based ceramic composition, B-site of which is partially substituted with (Co1/2W1/2). Cr2O3 increases a thermal stability of the piezoelectric composition. When more than 2 wt % of Cr2O3 is added to the main component, desired physical properties of the piezoelectric ceramic composition become poor because a second phase is formed. On the other hand, when less than 0.01 wt % of Cr2O3 is added to the main component, an increase in the thermal stability of the piezoelectric composition is negligible.
Furthermore, 0.1 to 0.5 wt %, preferably 0.3 to 0.4 wt % of MnO2 based on the total weight of the piezoelectric ceramic composition is added to the main component. When more than 0.5 wt % of MnO2 is added to the main component, desired physical properties of the piezoelectric ceramic composition become poor because a second phase is formed. On the other hand, when less than 0.1 wt % of MnO2 is added to the main component, a prevention of a leakage current cannot be sufficiently accomplished.
Tc is increased by partially substituting B-site of the PZT based composite oxide with (Co1/2W1/2). However, an increase of Tc cannot surely secure frequency stability of the piezoelectric ceramic composition. The increase of Tc can just slightly limit a variation in the frequency of the piezoelectric ceramic composition during heat treatment. To secure frequency stability, a specific additive is added to the main component.
Frequency stability is affected by a domain structure formed through a poling process. The domain structure is controlled by domain behavior, which is a variation of a microstructure and an inner crystal structure according to the kinds of additives added to the PZT based composite oxide. Accordingly, the domain behavior can be controlled by adding appropriate additives to the PZT based composite oxide, and thereby frequency stability can be secured.
As described above, 0.01 to 2.0 wt % of any one additive selected from the group consisting of CoO, MgO, ZnO, Al2O3, Fe2O3, Sb2O3, SnO2, CeO2, Nb2O5, V2O5 and WO3 is added to the main component expressed by Pb [(Co1/2W1/2)xTi1xe2x88x92xxe2x88x92yZry]O3 (wherein, 0.001xe2x89xa6xxe2x89xa60.04, and 0.35xe2x89xa6yxe2x89xa60.55). In consideration of variations in a frequency and an electrostatic capacity, it is preferable that CoO is used. For example, when less than 0.01 wt % of the additive is added to the PZT based composite oxide, the desired frequency stability cannot be obtained. On the other hand, when more than 2.0 wt % of the additive is added, desired physical properties of the piezoelectric ceramic composition become poor because a second phase is formed.
Based on Pb, Co, W, Ti, and Zr that are compounded within the above composition range, the ceramic composition of the present invention is useful as a material for piezoelectric devices such as filters, satisfying needed piezoelectric properties such as kp and Qm, as well as having a phase transition temperature of 320xc2x0 C. or higher, xc2x130 ppm/xc2x0 C. in TCF, and an oscillation frequency change of 0.1% or less after reflow. In addition, the piezoelectric ceramic composition further comprises Cr2O3 for improvement in thermal stability and MnO2 for reduction in leakage current. Upon polarization of the composition, the reduction in leakage current also results in a reduction in the dispersion attributed to nonuniform voltage, thereby increasing the production yield of devices from the composition.
As described above, the piezoelectric ceramic composition in which a B-site of ABO3 composite perovskite is substituted with cobalt and tungsten and additives such as Cr2O3, MnO2, etc. are added to the piezoelectric ceramic composition can be sintered under atmospheric pressure, and has excellent electric and thermal properties in consideration of a reflow process at 250xc2x0 C. or higher. That is to say, Fosc variation after reflow is 0.1% or less, a temperature coefficient of resonance frequency (TCF) is i 30 ppm/xc2x0 C., the phase transition temperature is 320xc2x0 C. or higher, and a leakage current is reduced in polarization.
Furthermore, a piezoelectric device using the piezoelectric ceramic composition has an excellent heat resistance and piezoelectricity, that is to say, both a frequency change and the leakage current after reflow are minimized.
Therefore, the piezoelectric ceramic composition of the present invention can be advantageously applied to an SMD type ceramic filter with superior area vibration properties, which can be laminated and allows the application of SMT thereto.
Illustrative, but non-limiting examples of the piezoelectric ceramic composition may include the piezoelectric ceramics, ceramic resonate, piezoelectric transducers, piezoelectric buzzers, and filters.